In a vehicle, particularly, an automotive vehicle having a monocoque body, with a view to enhancing strength of a rear of a vehicle body, a pair of right and left rear side frames each extending in a front-rear direction of the vehicle are commonly joined to a lower surface of a rear floor panel. Each of the rear side frames is formed in a cross-sectionally angular C shape opened upwardly, to form a closed cross-section in cooperation with the rear floor panel.
Absorption of collision energy during a rear collision of a vehicle (when the vehicle undergoes collision from behind) is primarily undertaken by such rear side frames. In order to effectively perform the absorption of collision energy during a rear collision, the following Patent Literature proposes to set an axial compression stiffness (rigidity) of a vehicle-widthwise outer portion of a rear side frame to become less than an axial compression stiffness of a vehicle-widthwise inner portion of the rear side frame, to thereby cause the rear side frame to be bent inwardly (to be deformed to protrude inwardly in the vehicle width direction) during a rear collision.
However, from the view point of energy absorption fully utilizing longitudinal deformation (deformation in the front-rear direction) of the rear side frame, it is undesirable to cause the rear side frame to be bent inwardly during a rear collision as described in the Patent Literature 1. Thus, there remains a need for improvement.
In order to effectively absorb collision energy during a rear collision by utilizing the longitudinal deformation of the rear side frame, it has been studied to form a bottom reinforcing portion in a bottom wall of the rear side frame to extend in the front-rear direction. The formation of the bottom reinforcing portion provides enhanced longitudinal strength (strength in the front-rear direction) of the rear side frame, so that it becomes possible to increase a maximum bending load (maximum reaction force), in a load-deformation characteristic indicative of a longitudinal deformation amount of the rear side frame depending on a magnitude of a load during a rear collision.
As the bottom reinforcing portion, it is conceivable to form a V-shaped protruding portion in a part of the bottom of the rear side frame. In this case, the bottom reinforcing portion is typically formed in a bilaterally symmetric shape with respect to a top (ridge) thereof at which a protruding amount is maximized. That is, the bottom reinforcing portion has an outer inclined section and an inner inclined section on respective right and left sides with respect to the top, wherein each of the inclined sections is formed to be inclined at the same angle.
In order to further enhance the collision energy absorption performance based on this rear side frame, it has also been studied to additionally form a side reinforcing portion protruding toward an inside of the cross-section of the rear side frame (protruding outwardly in the vehicle width direction), in an inner standing wall (standing wall located on a vehicle-widthwise inner side) of the rear side frame. The formation of the side reinforcing portion makes it possible to further enhance the maximum bending load of the rear side frame.
However, it was found that, in the case where both of the bottom reinforcing portion and the side reinforcing portion are formed in the above manner, although the maximum bending load of the rear side frame is significantly increased, a bending load after reaching the maximum bending load is not increased as large as expected. As a result of researches on the cause, it was found that, although the side reinforcing portion formed in the inner standing wall increases the maximum bending load, a bending load after reaching the maximum bending load is not sufficiently ensured due to buckling of the side reinforcing portion. That is, it was found that the bending load (reaction force) after reaching the maximum bending load is lower in the inner standing wall than that in an outer standing wall (standing wall located on a vehicle-widthwise outer side). Thus, after reaching the maximum bending load during a rear collision, the inner standing wall is more likely to undergo buckling by an inwardly-oriented component (component force oriented inwardly in the vehicle width direction) of a buckling force input into the inner inclined section of the bottom reinforcing portion. For this reason, the rear side frame is undesirably deformed inwardly (bent inwardly) in the vehicle width direction, resulting in failing to sufficiently ensure the bending load after reaching the maximum bending load.